A. Field of the Invention
This invention relates to a perpendicular recording discrete track medium, and to a servo pattern magnetization method for such medium.
B. Description of the Related Art
Since 1997, HDD (hard disk drive) recording densities have risen rapidly at an annual pace of between 60 and 100%. As a result of such remarkable growth, the in-plane recording methods which have been used in the past are approaching recording density limits. In light of this situation, much attention is focused on a the perpendicular recording method which enables still higher densities, and much research and development has been conducted. And, from 2005, HDDs adopting perpendicular recording have begun to be commercialized in some products.
A perpendicular magnetic recording medium mainly comprises a magnetic recording layer of a hard magnetic material; an underlayer, to orient the magnetic recording layer in a target direction; a protective film, which protects the surface of the magnetic recording layer; and a soft magnetic layer, which serves to concentrate magnetic flux generated by a magnetic head used in recording to the magnetic recording layer.
Research and development of discrete track media, in which a plurality of recording tracks are magnetically separated, is being performed with the aim of further increasing the recording densities of perpendicular magnetic recording media. In FIG. 5A, a schematic top view of a discrete track medium of the prior art is shown. The discrete track medium 500 is divided, in the circumferential direction on the disc-shaped substrate, into a plurality of sectors 530, each comprising recording areas 510 having a plurality of recording tracks for recording data and servo areas 520 having servo patterns for detection of the width-direction position of each track. In FIG. 5B, area 540 in a portion of sector 530 of the discrete track medium is shown enlarged and extended in a straight line, to show an example of the structure of recording areas 510 and servo areas 520. The plurality of recording tracks 512 in recording areas 510, as well as the plurality of servo blocks 522 holding position information for each track in servo areas 520, are formed as physical protrusions and depressions. Here, the plurality of tracks 512 and plurality of servo blocks 522 are formed by etching the magnetic recording layer using lithography techniques. The plurality of tracks 512 positioned at equal distances from the medium center together form concentric circular tracks 514. The plurality of tracks 512 are magnetically separated from adjacent tracks 512 by physical protrusions and depressions existing between the tracks and adjacent tracks 512, so that magnetic interference therebetween is mitigated.
An example of a servo pattern magnetization method for a discrete track medium of the prior art appears in FIG. 6. A discrete track medium generally has a structure comprising, at least, nonmagnetic substrate 610, soft magnetic layer 620, and magnetic recording layer 630 which is divided into a plurality of portions. In servo pattern magnetization, single-pole head 550 which imparts a magnetic field in the substrate perpendicular direction (here, an example is shown in which the S pole opposes the discrete track medium) is positioned in proximity to magnetic recording layer 630 of the discrete track medium, and is moved in the circumferential direction (track direction), as shown in FIG. 6A. The magnetization obtained by this process appears in FIG. 6B. A magnetization in a direction perpendicular to the substrate (here, upward) is imparted to both magnetic recording layers 630s (servo blocks 522) and 630r (tracks 512).
The process of information recording onto a discrete track medium is performed, as shown in FIG. 6C, by using the single-pole head 560 to impart a perpendicular-direction magnetization corresponding to the information (in both the upward and downward directions) to magnetic recording layer 630r (tracks 512). Signals obtained when a read head is used to read the magnetization of magnetic recording layer 630 appear in FIG. 6D. As is clearly shown in FIG. 6D, whereas data signals from recording areas 510 have full amplitude, servo signals from servo areas 520 have half amplitude.
When servo signals have half amplitude, the signal strength is half as great or less. As a result, the precision of head positioning is degraded, and higher track recording densities are difficult to achieve. And, because data signals have full amplitude, signal intensity modification and/or waveform shaping may be necessary when servo signals are read or when data signals are read.
In response to this problem, a method has been proposed in which servo patterns (position detection marks) arranged within servo zones comprise magnetic recording block areas capable of recording a plurality of bits, and full-amplitude servo signals (magnetization inversion signals) are recorded in the magnetic recording block areas (Japanese Patent Laid-open No. 2004-110896). In addition, a method has been proposed in which a perpendicular magnetic recording layer having two types of areas, with different coercivity, is used in servo signal areas (Japanese Patent Laid-open No. 2003-016623). In this method, initially a perpendicular magnetic field of sufficient intensity is applied, and the two types of areas are magnetized in a single direction. Then, a perpendicular magnetic field, of intensity sufficient in the opposite direction to invert the magnetization of only one among the two types of area, is applied, so that the magnetization is inverted in only one of the two types of areas, and servo signal areas are formed in which full-amplitude servo signals are recorded.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.